US3645657A - Method and apparatus for improved extrusion of essentially inviscid jets - Google Patents

Abstract

Low viscosity melts, notably metals and their alloys, are spun for extended periods, with attenuation and with induced velocity profile relaxation by spinning the molten materials at appropriate velocities first into a flowing inert gas and then into a film stabilizing gas. The inert gas zone is provided by a ''''gas plate'''' having an orifice aligned essentially coaxially with and beneath the extrusion orifice to provide an inert gas flow between the orifice plate and the gas plate.

Description

United States Patent Otstot et a1.

[54] METHOD AND APPARATUS FOR IMPROVED EXTRUSION 0F ESSENTIALLY INVISCID JETS Roger S. Otstot, Raleigh; John W. Mottern, Cary, both of NC.

Monsanto Company, St. Louis, Mo.

July 2, 1969 Inventors:

Assignee:

Filed:

Appl. No.:

US. Cl ..425/72, 164/66, 164/82, 425/461 Int. Cl. ..D0ld l/00, 322d 1 1/00 Field of Search ..164/81, 82, 66, 259, 273; 18/8 QD, 8 QM, 8 SC References Cited UNITED STATES PATENTS 2,879,566 3/1959 Pond ..l64/81X 2,976,590 3/1961 Pond ..164/82 3,048,467 8/1962 Roberts et al ..18/8 QM UX 3,061,874 11/1962 Lees ..l8/8 QM 3,516,478 6/1970 Dunn et al. ..164/82 X FOREIGN PATENTS OR APPLICATIONS 6,604,168 10/ l 966 Netherlands 164/82 Primary Examiner-R. Spencer Annear Attorney !ames W. Williams, Jr., Russell E. Weinkauf and John D. Upham [5 7] ABSTRACT Low viscosity melts, notably metals and their alloys, are spun for extended periods, with attenuation and with induced velocity profile relaxation by spinning the molten materials at appropriate velocities first into a flowing inert gas and then into a film stabilizing gas. The inert gas zone is provided by a gas plate" having an orifice aligned essentially coaxially with and beneath the extrusion orifice to provide an inert gas flow between the orifice plate and the gas plate.

3 Claims, 4 Drawing Figures is 1mm: j

METHOD AND APPARATUS FOR IMPROVED EXTRUSION F ESSENTIALLY INVISCED JETS FIELD OF THE INVENTION This invention relates to improvements in the formation of fibers and filaments by melt extrusion of essentially inviscid ets.

More particularly, the invention relates to the formation of fibers and filaments of materials which are essentially inviscid in the melt by extrusion of the molten essentially inviscid materials into atmospheres which stabilize the nascent molten fiber or filament prior to breakup caused by surface tension pending solidification.

BACKGROUND OF THE lNVENTlON Film stabilization of inviscid jets has been recognized recently as a practical means for forming filaments and fibers from materials which exhibit extremely low viscosities in the liquid or molten phase. Thus, where materials such as metals, metal alloys and ceramics exhibit viscosities in the molten phase of less than about poises and more commonly only a fraction of a poise, the surface tension of such a free molten filamentary stream is so great in relation to its viscosity that the stream tends to breakup into small spheres or shot before it can be solidified by cooling or quenching by practical means. It has been discovered that the length of the molten inviscid stream or jet, or the time in which such a jet exists as a continuous stream prior to breakup due to its surface tension when extruded at appropriate velocities can be considerably increased by extruding the inviscid jet into an atmosphere which upon contact with the nascent molten jet forms a thin film on the surface of the molten jet. The stabilizing film must, of course, be rapidly formed, be a solid or at least have a viscosity substantially greater than that of the molten jet and the film should be substantially insoluble in the molten jet under the conditions so that substantial, and desirably complete, continuity of the film is achieved and maintained. The means for film stabilizing inviscid jets are known and are varied as are the materials which may be stabilized. For example. the molten inviscid jet may be extruded into atmospheres which readily react with the surface of the molten jet to form a film or. the jet may be extruded into atmospheres which decompose upon contact with the molten jet to form films. Thus, a molten aluminum jet extruded into air is stabilized by the rapid formation of a film of aluminum oxide, which film is a solid at the optimum extrusion temperature and which film is substantially insoluble in the molten jet. Aluminum oxide jets, on the other hand, may be extruded into hydrocarbon atmospheres, such as propane, which upon contact with the hot ceramic jet decompose leaving a stabilizing carbon film on the jet. In a special case Alber et al. have noted in US. Pat. No. 3,216,076 that the oxides of certain metals, such as iron, silver and gold, are soluble in their respective metallic melts to the extent that they do not serve to form stabilizing films. Alber et al. suggest, therefore, that filaments can be formed from such materials by the film stabilized melt spinning technique by extruding alloys of such metals with compatible metals whose oxides are substantially insoluble in the molten jet. Thus, the jet of a ferrous alloy containing a small amount of a metal, such as aluminum, the oxide of which is insoluble in the jet can be effectively stabilized against surface tension promoted breakup, pending solidification by normal or even accelerated heat transfer phenomena.

Beyond the basic film stabilization phenomena, above discussed, there has been little or no recognition or resolution of the problems encountered by extrusion of molten essentially inviscid materials at extremely high temperatures to form shaped articles. The requirements of materials at very high temperatures and conditions for forming fibers and filaments from essentially inviscid materials in continuous processes are manifold and different to the extent that techniques known and used in the formation of fibers and filaments from viscous melts of glasses and synthetic polymers are generally not applicable.

it has been noted, for example, that velocity profiles tend to develop across the essentially inviscid jet or stream as it passes through the extrusion orifice. Upon issue from the orifice such velocity profiles then tend to relax or approach plug flow causing some change in shape of the still molten filamentary stream and, where a thin stabilizing film forms on the surface of the stream as a fragile cylinder about the molten jet prior to relaxation of the velocity profile, changes in shape of the stream due to velocity profile relaxation tend to rupture or break the film to thereby appreciably or wholly negate its in tended stabilizing function.

Another problem created by the essentially inviscid nature of the molten filamentary stream is that the met cannot be attenuated by drawing as in the case of viscous glassy and polymeric organic materials. Moreover, if means were discovered to effectively elongate the molten jet, the thin stabilizing film would be required to elongate proportionally, otherwise it would break thereby nullifying its effectiveness in stabilizing the stream. Yet attenuation is important in processes for spinning inviscid materials not only because of enhanced production capabilities but also because of the difficulty of making true fine diameter orifices in materials which are substantially inert at high temperatures and which orifice containing materials must be exceedingly strong at high temperatures to withstand extrusion pressure. Thus, where an orifice having a diameter of 20 mils in an orifice plate can be employed in the preparation of a 4 mil diameter filament, for example, the cost and ease of orifice preparation and orifice life are greatly improved.

A further and notable benefit which would result from stream attenuation lies in the greatly reduced pressure requirements necessary to cause the molten charge to flow through the orifice at a desirable extrusion velocity. Thus, where 1 mil diameter steel filaments are desired a pressure of about 400 p.s.i.g. is required to force the jet through a 1 mil diameter orifice at a given desirable velocity. Thus, the force exerted on the orifice is extremely high when considering that the thickness of the orifice plate would probably be less than 4 mils at the orifice. Where attenuation of the jet can be achieved a 1 mil fiber can be made from a 4 mil orifice, for example, in which case the pressure required to extrude the same mass per unit time as in the case of the 1 mil orifice would be substantially decreased.

Still another difficulty encountered in the film stabilization technique for producing metal and ceramic fibers involves stream deviation where the direction of the nascent molten stream tends to migrate away from the extrusion axis. This effect frequently involves undesirable stresses on the essentially inviscid liquidous portion of the stream and renders the control of tension and aerodynamic effects on the stream difficult, if not impossible. Deviation of the stream is believed to result from reaction of the stabilizing atmosphere with the molten jet at or within the extrusion orifice.

The use of film stabilization as a technique for stabilization of essentially inviscid jets requires extrusion into atmospheres which react rapidly with the surface of the molten jet. It has been observed that cylindrically shaped extrusion orifices are frequently nonful-running. That is, a vena contracta may occur within the orifice which provides a passageway for the film-forming atmosphere to enter the orifice. Growths, believed to be oxides, other reaction products or decomposition products of the film-forming atmosphere on the orifice plate, have been observed within the orifice and also as tubelike stalactites which grow from the extrusion orifice.

Growths within the orifice alter the extrusion orifice diameter and frequently cause the jet to veer from the path of the axis of the orifice. Usually such growths grow to the extent that they completely block the orifice. No less troublesome are the tubelike growths which grow from the orifice. Tubes, like internal growths, alter the course and diameter of the stream and, additionally, they alter the velocity profile of the stream insofar as a tube effectively changes the aspect ratio of the orifice. Mere blanketing the orifice with an inert gas has proved to be ineffective to prevent growths in processes for film stabilization of inviscid jets.

This invention, therefore, is concerned with melt extrusion apparatus which result in improvements in the formation of fibers and filaments of materials having very low melt viscosities.

This invention includes among its objectives apparatus for inducing velocity profile relaxation in a molten inviscid jet prior to the formation of stabilizing films.

Another object of this invention is the provision of apparatus for attenuating an essentially inviscid molten jet prior to film stabilization without application of substantial extraneous stresses downstream of the unstabilized liquidous region of the stream which extraneous stresses are transmitted to said unstabilized liquidous region.

A further object of this invention involves the substantial inhibition of growths within the orifice or at the orifice exit and the consequent reduction of deviation or migration of the molten stream from the path of the axis of the orifice.

BRIEF SUMMARY OF THE INVENTION The above objects of this invention have been accomplished by the continuous extrusion of molten essentially inviscid inorganic materials through an orifice as a free stream into a zone occupied by a flowing inert, gaseous atmosphere and then into a zone occupied by film-forming atmosphere as a continuous molten filamentary stream or jet. More particularly, the mo]- ten material is extruded through an orifice directly into flowing inert gas occupying a first zone immediately below the extrusion orifice which fist zone communicates with a second zone containing a film-forming atmosphere. The inert gas zone is connected to a source of inert gas which gas continuously passes through the first zone and flows out of the first zone through a second orifice or plate throat" which is essentially coaxial with the extrusion orifice and which communicates with the zone containing a film-forming atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself together with further objects and advantages may be best understood by reference to the following description taken with the accompanying drawings in which:

FIG. I is a cross-sectional schematical view of a typical assembly for spinning essentially inviscid molten materials;

FIG. 2 is a cross-sectional schematical view of an orifice assembly in accordance with the teachings of the present invention;

FIG. 2a is a plan view of the orifice assembly of FIG. 2; and

FIG. 3 is an enlarged view of the orifice assembly of FIG. 2.

DETAILED DESCRIPTION FIG. 1 depicts a source of a molten essentially inviscid material 1 under positive pressure supplied to an extrusionorifice 2 in a crucible 3. The molten essentially inviscid filamentary stream or jet 4 issues from orifice 2 in orifice plate and passes through inert gas zone or gap 5 formed by the parallel arrangement of the gas plate 6 with the orifice plate 10. Inert gas may be passed into zone 5 through port 7 shown in pedestal II. The inert gas then flows through a passage provided by the gas plate throat 8, essentially coaxially aligned with orifice 2, into a second zone containing a film-stabilizing atmosphere contained in chamber 9.

Generally, the formation of fibers and filaments by the film stabilized inviscid spinning process is applicable to the extrusion of jets having diameters of less than about 50 mils. It appears that above about 50 mils sufficient transfer of heat out of the molten stream, even though film-stabilized, is difficult to accomplish as a practical matter to prevent breakup even when the jet is extruded into a cooled chamber. Moreover, where large diameter jets are extruded it appears that the momentum of the stream is sut'ficient to remove tubes and other orifice obstructions above-noted. On the other hand, when using the film stabilization technique fine diameter fibers can adequately cool prior to breakup at room temperature or greater so that there is no necessity for elaborate cooling systems for chilling or attempting to supercool the molten jet. In order to provide sufficient jet lengths initially to provide for film stabilization of the molten filamentary shaped jet the velocity of extrusion in a given case should be such that the Rayleigh parameter, (Ra), a dimensionless quantity,

lies between 1.5 and 25, where Vis the jet velocity (cm./sec.), D is the jet diameter upon issue (cm.), p and 'y the melt density (gm/cm?) and surface tension (dynes/cmF), respectively, of

the molten material. Where the velocity is such that the Rayleigh parameter falls below about l.5 the jet length may be so short that it normally cannot be adequately stabilized prior to breakup. Conversely, where the velocity of the molten jet is too high breakup can be caused by aerodynamic deceleration.

As earlier indicated according to this invention the molten essentially inviscid jet is extruded directly into a flowing inert gaseous atmosphere which is supplied to the zone between the orifice plate and the gas plate through the passage provided by the gas plate orifice or throat and into the film-stabilizing atmosphere where a stabilizing film is formed and the molten stream is solidified by cooling prior to breakup. The nature of the inert gas does not appear to be critical as long as the gas in inert to the extruded materials, the orifice plate and other parts of the extrusion apparatus. Helium and argon have been successfully employed and they may contain other ingredients which would inhibit growths in the inert gas zone without attacking the molten jet or the extrusion apparatus. In dealing with inert gases from commercial sources it is usually necessary to treat the gas to remove minor impurities, such as oxygen, which, even in quantities as high as one part per million become quite reactive with the met or parts of the extrusion apparatus at highly elevated temperatures. Thus, the term, inert gas," is intended to connote gases having constituents reactive at extrusion temperatures with the molten jet or apparatus in concentrations of less than about one part per million.

A principal requirement of this invention is that the inert gas velocity in the gas plate throat lies above a minimum value and that the upper velocity of the gas be such that it does not cause the unstabilized melt to break up into shot. This velocity can readily be regulated by the amount of gas supplied to the inert gas zone in relation to the diameter of the gas plate throat.

The successful operation of this invention requires that the velocity of the gas through the throat of the gas plate be maintained above a certain minimum for any given system to preclude diffusion of the film-forming gas into the inert gas zone. As a practical matter the gas flow may be measured by a rotameter placed between the inert gas source and gas plate. Thus, the volume flow rate or the quantity per unit time of inert gas supplied to the gas plate, 0, measured on the rotameter or other suitable device at 25 C. must be at least the value wherein K., (Kf) is at least 8 and preferably 12, T is the temperature of the gas passing through the gas plate (K.), X (cm.) is the distance from the extrusion orifice to the second zone, normally the length traversed by the jet in the gap plus the length of the gas plate throat, A is the minimum cross-sectional area (cm?) of the plate throat, A is the molten jet cross-sectional area (cm?) passing through the throat and Dx is the diffusivity (cc/sec.) of a film-forming gas through an inert gas in the system. While spinning runs of substantial periods without stream deviation and blockage can be performed where K. is 8,.values of K. of at least 12 are normally required where continuous spinning runs are desirable.

The upper limit for the amount of inert gas passed through the plate is simply that amount which causes disruption and formation of powder or shot from the nascent inviscid filamentary stream.

Generally speaking the gas entering the chamber becomes heated as it passes to a gas distribution ring which may conveniently be coaxial with the orifice and gas plate throat. The gas may be distributed radially or in a direction normal to the met suchthat there is substantial symmetry of flow. This flow is in large measure self-distributing toward symmetrical flow. The inert gas flow within the limits described serves to maintain the molten jet in a predetermined path and precludes diffusion of film-forming gases into the orifice throat. The process of this invention can, if desired, be employed to produce discontinuous fibers, i.e., those having a significant aspect ratio, for example an aspect ratio greater than five, as opposed to either shot or continuous lengths. Thus, it has been observed that as the inert gas velocity is increased beyond the velocities which can result in continuous filament formation, there is an upper velocity region in a given system where short fibers are produced. If the velocity of the gas is increased beyond the short fiber-forming region of the extruded produce then becomes a fine powder, commonly known as shot.

H68. 2 and 2A illustrate a typical gas plate 6 from vertical cross section and top elevation, respectively, wherein an inert gas entering port 7 circulates in gas distribution ring 12, passes across land 13 and out of the gas plate throat 8. Other gas distribution means have been successfully employed and except as hereinafter described in greater detail the particular geometry of the gas plate has not been found to be a critical feature of this invention.

It has been observed that velocity profiles which apparently develop from shear forces on the jet within the extrusion orifice are relaxed by passage of the molten unstabilized jet,

through the gas plate throat. As earlier mentioned velocity profile relaxation of film stabilized molten inviscid jets presents a considerable barrier to the basic nature of film stabilization of inviscid jets insofar as velocity profile relaxation can effectively destroy stabilizing influence of the film. Although velocity profiles can be reduced from parabolic flow by resort to short bore or knife-edge orifice configurations, the use of such configurations, in turn, causes problems, particularly in the use of fine diameter orifices at temperatures around 1,000 Ciand greater, because the pressures necessary to extrude the melt from such orifices place unusually high mechanical stresses on the thin portion of the orifice plate. Moreover, severe erosion and limited orifice life, even in the strongest materials, renders the use of short bore or knife edge orifices uneconomic as a practical matter. According to the instant invention relaxation of velocity profiles can be induced after extrusion of the nascent jet and prior to film stabilization to thereby accommodate the use of long bore orifices (i.e., those having an aspect ratio of greater than about 4), thereby, in turn, making provision for strong orifice plates and reducing the material requirements for high temperature extrusion. The flow gas through the gas plate within the limitations of this invention can be regulated for any given orifice and extruded material to accommodate velocity profile relaxation prior to film stabilization.

While the invention as above-described can be successfully employed to correct stream deviations and relax velocity profiles prior to film stabilization, the process of this invention may, additionally and advantageously, be employed to attenuate the nascent jet prior to film stabilization. The term, attenuation," as herein employed means a reduction in the diameter of the met. Reduction in diameter, in turn, results in a higher attenuated jet velocity. in the synthetic fiber and glass fiber arts attenuation is classically achieved by stretching the filamentary mass while in a highly viscous condition. However, when metals and other essentially inviscid inorganic melts are extruded from an orifice in molten filamentary form there is a comparatively sharp zone of solidification. Any attempt to substantially stretch the filament at or below the zone of solidification results in complete disruption of the fragile liquidous part of the stream. It has now been discovered that through the use of the method and apparatus herein described essentially inviscid jets can be attenuated without the application of substantial extraneous downstream pull. Thus, according to the instant invention an inviscid jet may be attenuated by extrusion of the melt directly into an inert gas and then into a film-forming atmosphere under conditions which establish a pressure gradient between the extrusion orifice and the area principally occupied by the film-forming atmosphere. Thus, attenuation is achieved where the pressure in the inert gas zone is less than the pressure exerted on the melt in the orifice and greater than the pressure in the film-forming zone beneath the inert gas plate. The degree of attenuation may be varied by variations in the pressure gradient; the pressure gradient in turn being varied by the velocity and density of the inert gas in the inert gas zone having a given geometry as hereinafter more fully described. While the theory of attenuation of inviscid jets by the means herein described is not fully understood, it has been found as a practical matter that attenuation results where there exists such a pressure gradient. The pressure gradient may be determined by simply placing a pressure gauge on the extrusion orifice in a blank run or by converting reduced mass flow resulting from back pressure in pressure units. The degree of attenuation can be ascertained by comparison of a product extruded from a given orifice diameter under normal conditions with a wire product extruded from an orifice of the same diameter through the pressure gradient as above defined. The benefits of the capability to attenuate the jet in an inviscid spinning or extrusion operation are many both with respect to economic and technical considerations. Thus, steel wire having a 3 mil diameter can be produced with attenuation by extrusion from a 6 or 9 mil orifice, for example. Insofar as the pressure required to extrude a unit mass of steel from a 3 mil orifice is far greater than that required to extrude the same amount from a 6 or 9 mil orifice attenuation results in greatly reduced extrusion pressure requirements. Reduced extrusion pressure requirements, in turn, reduce the high temperature strength requirements of the orifice plate. Furthermore, an orifice having an aspect ratio of five, for example, would be only 15 mils thick, whereas, a plate having a 9 mil orifice would 45 mils thick using the same aspect ratio. Thus, for a given rate of production of wire of a given diameter attenua tion beneficially results in reduced extrusion pressures along with thicker (and therefore stronger) orifice plates thereby reducing the high temperature strength requirements of orifice plate construction materials. Additionally, it has been found that great savings can be realized through the use of larger orifices because they are much simpler to fabricate. Moreover, tolerances in larger orifices are greater than in smaller orifices.

From the foregoing discussion it will be readily apparent that the use of the gas plate beneficially provides the artisan with a ready means for controlling fiber diameter.

The apparatus employed in this invention comprises a first plate defining an extrusion orifice and a second plate defining a second orifice coaxial with said extrusion orifice and having a diameter at least as large as said extrusion orifice, said first and second plates defining an enclosed chamber, means for continuously supplying a molten material first through said extrusion orifice into the chamber and means for supplying an inert gas into the chamber.

In a preferred embodiment the apparatus employed in this invention comprises a parallel arrangement of a first plate or orifice plate and second plate or gas plate defining a first and a second orifice, respectively, each orifice being essentially coaxial with an axis normal to the transverse planes of said first and second plates, said first and second plates defining an enclosed chamber, means for supplying an essentially inviscid melt for extrusion through said first orifice into said chamber As above-indicated there are practical working relationships in the geometry of the apparatus of the instant invention. FIG. 3 is a schematic of a vertical cross section which illustrates the relationship of the orifice plate 10 to the gas plate 6. As earlier set forth the operation of this invention requires concommitant molten jet and inert gas flow through the gas plate throat. Thus, the minimum diameter of the gas plate throat must at least be large enough for the molten filamentary stream and inert gas to pass through the throat. The diameter of the gas plate throat is also limited to the diameter at which the inert gas can be made to flow through at the required rates. As a practical matter very large diameter plate throats are undesirable because the volume of inert gas entering the second zone makes if difficult to enable contact between the film stabilizing gas and the molten free stream in the second zone. In order to obtain high gas velocities through the gas plate throat using reasonable volumes of inert gas its minimum diameter 15 should lie below thirty times and preferably below 10 times the extrusion orifice diameter 14. Additionally, the gap 16 in the inert gas zone 5 should be less than 15 times the orifice diameter 14 and preferably less than one-half the diameter of the gas plate throat 15. The length of the gas plate throat is normally maintained at less than about 100 times and preferably less than 50 times the orifice diameter.

While the drawings and discussions herein relate to certain preferred and simplified gas plate geometries other arrangements may be employed. For example, plate throat 8 may be designed as a truncated cone the theoretical apex of which may lie either toward or away from the extrusion orifice.

The combination of the gas plate and orifice plate may be assembled in a variety of ways. For example the orifice plate and gas plate may be separate members inserted in a crucible baseplate. The assembly may be formed by machining the inert gas port, plate gap and plate throat in a crucible baseplate and inserting thereon an insert defining an extrusion orifice. Another variation of the assembly comprises machining the gap in the plate defining the extrusion orifice and fitting a flat plate thereunder defining a gas plate throat. Other variations are possible and will bereadily apparent to those skilled in the art. The materials from which the orifice plate and the gas plate are constructed should be essentially inert, each to the other, under the conditions of the extrusion process. They may, of course, by made from the same material. Moreover, the materials should be selected such that they are, desirably inert to the molten material and so far as is practical, resistant to thermal shock and possess the mechanical strength to withstand the stresses to which they are put. For example, in the extrusion of metals such as copper and ferrous alloys, ceramics such as high density alumina, magnesia, thoria, beryllia and zirconia are useful materials for construction of the apparatus herein described. For high temperature extrusion processes using ceramic charges materials such as molybdenum and. graphite can be employed. For extrusion processes involving lower temperatures, stainless steel assemblies have been found to perform well. Other materials and combination of materials may be employed within the practice of the instant invention.

The invention is particularly applicable to melt extrusion of low viscosity inorganic materials by film stabilization such as described hereinbefore which and includes the extrusion of a variety of metals and their alloys such as lead, tin, copper, aluminum, iron and alloys thereof, including stainless steels, carbon steels and others. Additionally the process is also applicable to the extrusion of a variety of ceramic compositions and metalloids which are essentially inviscid in the molten phase and which cannot be extruded by conventional polymer and glass extrusion techniques.

The following examples are provided to illustrate results achieved through the use of the process of the invention and are not intended to limit the invention.

EXAMPLE 1 A melt spinning apparatus comprising a crucible having an extrusion orifice and a gas plate situated therebeneath and having a gas plate throat coaxial with the extrusion orifice was employed to spin a lead/tin alloy. The extrusion orifice was a straight bore orifice 4 mils in diameter and 4 mils long. The gas plate provided a gap of 7 mils beneath the lower extremity of the extrusion orifice. The gas plate throat was 13 mils in diameter and 7 mils in length.

The molten lead/tin alloy (62/38 weight at 300 C. was forced through the extrusion orifice by a 20 p.s.i.g. head pressure directly into the gap between the orifice and the gas plate occupied by flowing helium gas and then concurrently with helium through the plate throat and then into air. The flow rate of helium was measured at 250 cc./min. and although the gas was not preheated its temperature rose to C. through contact with the molten stream and elements of the assembly. The molten jet remained continuous and did not deviate from a straight path over extended periods of continuous spinning. With 7 other conditions remaining the same the stream underwent severe deviation resulting in repeated fiber discontinuities shortly after the helium flow rate was reduced. Visual examination of the orifice after shutdown revealed macrogrowths formation at the exit of the extrusion orifice indicating that the reduced helium flow had not prevented growths on the orifice.

EXAMPLE ll Example I was repeated except that the temperature of the molten alloy in the crucible was increased to 430 C., and the diameter of the plate throat was increased to 18 mils. Continuous, undeviated streaming of the molten jet was achieved at a helium flow rate of 630 cc./min. and above.

EXAMPLE [I] Example I was repeated using an extrusion orifice having a diameter of 4 mils and an aspect ratio of 6. The gas plate throat had a diameter of 15 mils, a throat length of 20 mils and a gap between the extrusion orifice and the plate of 15 mils. Helium flow rates were incrementally increased as indicated in Table l and resulted in corresponding increased jet attenuatron.

Neither discontinuities, stream deviations nor orifice growths were noted to result from the experiments above noted when steady flow rates at the indicated levels were maintained.

EXAMPLE lV Under a pressure of 40 p.s.i.g. molten commercial grade 2024 aluminum heated to 730 C. was extruded as a free molten stream from a 4 mil diameter orifice having an aspect ratio of 6 into flowing helium and then into air. The gap between the gas plate and the extrusion orifice was 10 mils. The plate throat had a diameter of 18 mils and a length of 20 mils. Helium flow rates of 600 cc./min. resulted in uninterrupted continuous jets streaming for more than 6 hours. Reduced helium flow rates resulted in the formation of growths at the exit of the extrusion orifice after relatively short streaming periods.

EXAMPLE V EXAMPLE V1 Commercial grade 2024 aluminum alloy was spun as a continuous filament from a melt at 720 C. into flowing helium and then into air. The extrusion orifice was 8 mils in diameter .and 48 mils in length. The plate defining the gas zone was situated 7.5 mils beneath the orifice and provided an orifice 18 mils in diameter and 22.5 mils in length, coaxially aligned with the extrusion orifice. Table 11 reflects varying degrees of attenuation of the molten free stream prior to solidification principally as a function of varying helium flow rates.

in the experiments reported in Table 11 there existed a back pressure at the extrusion orifice caused by the high velocity flow of helium through the inert gas zone. The existence of the back pressure at the extrusion orifice results in corresponding reduction in the effective overhead extrusion pressure.

EXAMPLE V11 Commercial grade 2024 aluminum was spun as a molten free stream from an 8 mil diameter orifice (aspect ratio of 4) into an inert gas zone defined by the orifice plate and a gas plate, through the gas plate throat and into air. The gas plate was 7.5 mils beneath the orifice plate. The throat was 25 mils in diameter and 22.5 mils in length. Several runs were conducted employing extrusion pressures and helium flow rates indicated in Table 111.

TABLE 111 Extrusion He Flow Fiber Diameter Diameter Pressure Rate (cc/min.) (mils) Reduction(%) (p.s.i.g.)

9 3390 4.9s 33.0 1 3190 est 39.5

EXAMPLE V111 Using an orifice having a 4 mil diameter and an aspect ratio of 8 in combination with a gas plate having a throat diameter of 18 mils and a throat length of 22.5 mils to provide a gap of 7.5 mils, molten 2024 aluminum at 720 C. was extruded as a free stream directly into flowing helium, through the gas plate throat and into air under the conditions indicated in Table IV.

TABLE [V Extrusion He Flow Fiber Diameter Diameter Pressure Rate (cc.lmin.) (mils) Reduction (P- -B-) In the several experiments noted in Example Vll and VIII growths were not observed to form at or within the orifice even after extended periods of extrusion. in each case there existed a back pressure at the orifice (observed by slightly diminished mass flow through the orifice) resulting in attenuation of the molten free stream prior to solidification.

Although relaxation of velocity profiles caused by spinning through an orifice having an aspect ration of 8 normally results in disruption of the stabilizing film which, in turn, results either in stream disruption or fibers having spaced modules, smooth fibers were produced in the runs reported in Example V111 indicating that the flow of helium along with the molten stream through the gas plate throat induces relaxation of velocity profiles prior to stabilization.

We claim:

1. An improved orifice assembly for the formation of fibers and filaments from an essentially inviscid melt by extrusion thereof through an orifice in an extrusion orifice plate as a free molten filamentary stream in a film-forming atmosphere whereby the stream is maintained in filamentary form by a film until solidified, the improvement comprising a. a second plate positioned beneath said orifice plate and having an orifice substantially coaxial with said extrusion orifice, said orifice plate and second plate defining an enclosed chamber having a gap distance therebetween in the vicinity of said orifices less than one-half the diameter of said second plate orifice, said second plate orifice further having a length less than times the diameter of said extrusion plate orifice and a diameter less than 30 times the diameter of said extrusion plate orifice; and

b. port means communicating with said enclosed chamber for providing a predetermined flow of inert gas from said port means into said second plate orifice thereby providing a blanket of inert gas about the molten stream as it passes through said enclosed chamber and said second plate orifice into the film forming atmosphere.

2. The assembly of claim 1 wherein the diameter of said second orifice is less than 10 times the diameter of said first orifice.

3. The assembly of claim 1 wherein the length of said second orifice is less than 50 times the diameter of said first orifice.

Claims (3)

1. An improved orifice assembly for the formation of fibers and filaments from an essentially inviscid melt by extrusion thereof through an orifice in an extrusion orifice plate as a free molten filamentary stream in a film-forming atmosphere whereby the stream is maintained in filamentary form by a film until solidified, the improvement comprising a. a second plate positioned beneath said orifice plate and having an orifice substantially coaxial with said extrusion orifice, said orifice plate and second plate defining an enclosed chamber having a gap distance therebetween in the vicinity of said orifices less than one-half the diameter of said second plate orifice, said second plate orifice further having a length less than 100 times the diameter of said extrusion plate orifice and a diameter less than 30 times the diameter of said extrusion plate orifice; and b. port means communicating with said enclosed chamber for providing a predetermined flow of inert gas from said port means into said second plate orifice thereby providing a blanket of inert gas about the molten stream as it passes through said enclosed chamber and said second plate orifice into the film forming atmosphere.

2. The assembly of claim 1 wherein the diameter of said second orifice is less than 10 times the diameter of said first orifice.

3. The assembly of claim 1 wherein the length of said second orifice is less than 50 times the diameter of said first orifice.

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